1. Field of the Invention
The present invention relates to an aluminum alloy brazing sheet for vacuum brazing which exhibits excellent corrosion resistance and is used for heat exchangers such as an evaporator or an intercooler for an air conditioner used in automobiles. The present invention also relates to a heat exchanger using the brazing sheet.
2. Description of Background Art
A heat exchanger made of an aluminum alloy has been used for automotive heat exchangers such as an oil cooler, inter cooler, heater, evaporator for an air conditioner, and a condenser, and an oil cooler for hydraulic devices and industrial machines. The structure of such a heat exchanger has been modified in various ways. For example, for an evaporator and a condenser, a corrugated-fin type heat exchanger fabricated by brazing a corrugated brazing sheet fin material and a serpentined porous extruded flat tube has mainly been utilized. At present, a drawn-cup type heat exchanger exhibiting improved heat exchanging performance is widely used for an evaporator. Such a drawn-cup type heat exchanger is fabricated by forming a fluid passage between laminated core plates produced by press-forming a brazing sheet clad on both sides thereof with a brazing material, laminating a corrugated fin material made of an aluminum alloy on the core plates, and brazing these materials.
As a core plate for a drawn-cup type heat exchanger, a brazing sheet consisting of a core material which comprises an aluminum alloy such as an Alxe2x80x94Mn, Alxe2x80x94Mnxe2x80x94Cu, Alxe2x80x94Mnxe2x80x94Mg, Alxe2x80x94Mnxe2x80x94Cuxe2x80x94Mg aluminum alloy containing Mn as an essential component, such as a JIS3003 alloy or 3005 alloy is used. Either one or both sides of the core material is clad with an Alxe2x80x94Sixe2x80x94Mg filler material. As a fin material, an Alxe2x80x94Mn, Alxe2x80x94Mnxe2x80x94Cu, Alxe2x80x94Mnxe2x80x94Mg, or Alxe2x80x94Mnxe2x80x94Zn aluminum alloy is used. The core plate and the fin material are generally joined by vacuum brazing.
However, the core material of the above brazing sheet, which comprises Mn as an essential component, exhibits inadequate pitting-corrosion resistance. Therefore, when applied to a fluid passage for a coolant, perforation leakage may occur due to pitting corrosion from the outer surface (surface in contact with air). To deal with this problem, a method of using a fin material having a potential less noble than that of a fluid passage material such as an Alxe2x80x94Mnxe2x80x94Zn alloy or an Alxe2x80x94Mnxe2x80x94Sn alloy has been proposed to prevent the fluid passage material from being corroded by the sacrificial anode effect of the fin material. However, this method has the following problems when applied to an evaporator. Specifically, when a coolant in a heat exchanger evaporates to absorb heat from the air, the temperature of the surface of the heat exchanger decreases and causes dew to form on the fin. This dew has a significantly low electrical conductivity because of the low impurity content. A corrosion resistant current generated by the sacrificial anode effect of the fin reaches only to near the joint section of the fin and no sacrificial anode effect occurs in the area apart from the joint section. Moreover, SO42xe2x88x92, NO3xe2x88x92, or the like contained in exhaust gas of automotive fuel such as diesel fuel or gasoline or emitted from factories is mixed in the dew to concentrate in the pitting corrosion. This significantly promotes corrosion, to cause perforation corrosion in the core plate, whereby a coolant tends to leak.
To solve these problems, a quad-layer clad material has been proposed (Japanese Patent Application Laid-open No. 60-251243). In the quad-layer clad material, an intermediate layer material made of an aluminum alloy having a potential 20-100 mV less noble than that of the core material is provided between the core material and the filler material. However, although the pitting-corrosion resistance is improved in this quad-layer clad material, it is difficult to ensure pressure resistance for heat exchangers due to the inferior strength after brazing. Moreover, joining between each layer during hot rolling tends to be inadequate. Furthermore, the quad-layer clad material tends to warp due to the nonuniform strength and elongation between each layer. In addition, an uneven cladding rate hinders fabrication. In order to improve pitting-corrosion resistance, the addition of Ti, Cu, Mg, and Si to an Alxe2x80x94Mn alloy constituting a core material of a core plate material for a drawn-cup type heat exchanger has been proposed. The addition of these elements forms a structure exhibiting periodic differences in the Ti concentration in the direction of the plate thickness. This material exhibits good corrosion resistance in a region where there are many sea salt particles. However, in a region where the amount of exhaust gas is too great, the effect is inadequate.
A method of adding Zn to the brazing material of the above core plate material has been proposed. This method is intended to provide corrosion resistance by the sacrificial anode effect of Zn. Zn is effective as a sacrificial anode. However, since the vapor pressure of Zn at the brazing temperature is higher than the degree of vacuum in a brazing furnace, the Zn almost all evaporates during vacuum brazing and does not remain in the core plate material, whereby no sacrificial anode effect occurs. Another method which comprises coating an evaporator with Zn after brazing and diffusing the Zn by heating has also been proposed. However, this method increases the number of fabrication steps and the cost. When providing an intermediate layer material between the core material and the filler material, the difference in the strength or elongation between the intermediate layer material and the core material causes the clad material to warp during hot rolling or results in inadequate joining between each layer during clad rolling. Moreover, the clad thickness may become uneven. Warping can be reduced to some extent by decreasing the rolling rate. However, this decreases production efficiency and is not suitable for mass production.
Furthermore, penetration of melted braze into the core material has been pointed out as a problem. In the case of a drawn-cup type heat exchanger, a fluid passage is formed by press-forming annealing materials (full-annealed materials) used as core plate materials and joining the annealing materials by brazing. The strain introduced during press forming recrystallizes and disappears during brazing. However, in the case of an Alxe2x80x94Mn alloy, if the amount of strain is small, the strain does not completely disappear at the brazing temperature due to the increased recrystallization temperature, whereby subgraining formed by the recovery of the strain remains. Because diffusion easily occurs at the subgrain boundary, a large amount of Si filler diffuses into the core material, thereby resulting in lack of filler. The area where the braze penetrated exhibits inferior corrosion resistance and decreased strength. To solve this problem, a method of homogenizing the core material at a high temperature, or a method of performing final annealing at a high temperature for a long period of time has been proposed. However, these methods are still inadequate.
In order to obtain a quad-layer clad aluminum alloy brazing sheet for vacuum brazing, which comprises a core material, intermediate layer material, and filler material on both sides of the core material, and can solve the above problems in a fluid passage material of an aluminum alloy heat exchanger, the present inventors have conducted extensive studies on the composition of the intermediate layer material and properties of the core material to provide excellent corrosion resistance and to solve problems in the fabrication of a clad material. As a result, the inventors have found that the following phenomena (1) to (4) occur by (a) limiting the Fe content in the intermediate layer material, (b) adding Zn to the intermediate layer material, (c) adding In and Sn to the intermediate layer material, and (d) allowing a predetermined amount of strain to remain in the core material.
(1) Si in the brazing material joined on both sides of the brazing sheet diffuses into the intermediate layer material and the core material during heating for brazing. The diffusion rate of Si increases particularly at the crystal grain boundary and therefore Si preferentially penetrates this area. As a result, the melting point decreases near the crystal grain boundary of the intermediate layer material, thereby causing local melting and increasing the amount of an Si solid solution. Moreover, Cu in the core material diffuses into the intermediate layer material in an amount greater than the diffusion between the solid phases. This decreases the sacrificial corrosion resistance effect of the intermediate layer material, whereby the intermediate layer material is perforated at an early stage. However, the grain size of the intermediate layer material becomes coarse after being heated for brazing by limiting the Fe content and the Si content in the intermediate layer material. This decreases the line length of the grain boundary per one cross section, which is the Si diffusion site. Because of this, the intermediate layer material having superior sacrificial corrosion resistance properties can be ensured over a wide area, thereby remarkably improving perforation durability.
(2) In the case of adding Zn to the intermediate layer material, Zn diffuses into the filler material near the surface of the quad-layer clad material and evaporates during brazing. However, Zn remains at the side of the core material and diffuses into the core material. Therefore, the Zn concentration peaks near the interface between the intermediate layer material and the core material and decreases at the side of both the filler material and the core material. Because of this, the electric potential of the quad-layer clad material becomes the lowest potential near the interface between the intermediate layer material and the core material, and gradually becomes noble toward the inner side of the core material. Therefore, the intermediate layer material and the core material on the side of the intermediate layer material act as a sacrificial anode for the core material on the side of the filler material, thereby preventing perforating corrosion. The addition of In and Sn to the filler material further improves corrosion resistance, since the brazing material also acts as a sacrificial anode.
(3) In the case of adding In and Sn to the intermediate layer material, In and Sn diffuse into the filler material near the surface of the quad-layer clad material, but scarcely into the core material. In and Sn diffused into the filler material do not evaporate and remain therein. Therefore, the concentration of In and Sn are uniformly distributed over almost the entire section of the intermediate layer material. The intermediate layer material and the core material in contact with the intermediate layer material act as a sacrificial anode to prevent perforating corrosion. In the case of adding In and Sn to the filler material, the filler material exhibits a significant sacrificial anode effect.
(4) It is important to control precipitated grains of the core material to improve brazability (erosion resistance). Moreover, it is considered to be advantageous to allow the strain to remain in the core material and to recrystallize during heating for brazing into a state in which the sub-structure does not remain. However, when the strain is allowed to remain in the core material, if the amount of strain is too great, cracking occurs during press forming. If the amount of strain is too small, erosion occurs during heating for brazing, thereby resulting in decreased brazability. Therefore, it is difficult to ensure both the press-formability and erosion resistance at the same time. The present inventors have found that the amount of the strain remaining in the core material can be quantified and the range thereof can be limited by forming a quad-layer structure, making the intermediate layer material into a full-annealed state, and limiting the ratio of the micro-Vickers hardness of the core material in a raw material state to that of the core material in a full-annealed state, whereby press-formability as well as erosion resistance can be ensured.
The present invention has been achieved based on the above findings. An object of the present invention is to provide an aluminum alloy brazing sheet for vacuum brazing which excels in corrosion resistance, clad rolling characteristics, and formability, and a heat exchanger using the aluminum alloy brazing sheet.
In order to achieve the above object, an aluminum alloy brazing sheet for vacuum brazing exhibiting excellent corrosion resistance of the present invention is characterized as follows.
1. An aluminum alloy brazing sheet with a quad-layer structure consisting of an outer filler material, intermediate layer material, core material, and inner filler material clad in that order from the outer surface, wherein the core material comprises 0.5-1.6% of Mn, 0.10-0.50% of Cu, 0.05-0.50% of Mg, and 0.06-0.30% of Ti, and, as impurities, 0.5% or less of Fe, 0.5% or less of Si, and 0.1% or less of Zn, with the remainder consisting of Al and unavoidable impurities; the intermediate layer material comprises 0.2-1.5% of Mg and at least one of 0.5-4% of Zn, 0.005-0.2% of In, and 0.01-0.2% of Sn, and, as impurities, 0.3% or less of Si, 0.3% or less Fe, 0.05% or less of Cu, 0.05% or less of Mn, and 0.3% or less of Ti, with the remainder consisting of Al and unavoidable impurities, wherein the thickness of the intermediate layer material is 50 xcexcm or more and is equal to or less than the thickness of the core material (thickness of the intermediate layer material/thickness of the core material xe2x89xa61); and the outer filler material and the inner filler material are Alxe2x80x94Sixe2x80x94Mg alloys.
2. In the aluminum alloy brazing sheet according to the above 1, the outer filler material further comprises at least one of 0.005-0.2% of In and 0.01-0.2% of Sn.
3. In the aluminum alloy brazing sheet according to the above 1 or 2, the average grain size of the intermediate layer material is 60 xcexcm or more.
4. In the aluminum alloy brazing sheet according to any one of the above 1 to 3, the average grain size of the intermediate layer material after heating for brazing is 80 xcexcm or more.
5. In the aluminum alloy brazing sheet according to any one of the above 1 to 4, the intermediate layer material further comprises Zn, wherein the Zn concentration at the interface between the intermediate layer material and the core material after heating for brazing is 0.1-2.0%, and the Zn concentration in the core material from the center of the plate thickness to the interface between the core material and the filler material is 0.3% or less.
6. In the aluminum alloy brazing sheet according to any one of the above 1 to 5, the ratio of the hardness of the core material to that of the core material in a soft state is 1.15-1.75.
A heat exchanger according to the present invention is fabricated by using the brazing sheet according to any one of the above 1 to 6, wherein the intermediate layer satisfies either the above 4 or the above 4 and 5 over the entire section.